Gas turbine engines, particularly those used for aircraft propulsion, must be capable of being reliably started under a wide variety of environmental and operational conditions. For example, ground starts of an aircraft engine may take place at airports whose elevations range from approximately sea level to over 14000 feet. In addition, it is desirable to restart an engine which has become temporarily disabled in flight. These in-flight starts, customarily referred to as airstarts, are conducted while the aircraft is moving forward at a considerable velocity and at altitudes which may exceed 40000 feet.
The ability to reliably start an aircraft engine is important to aircraft operators and airport operators alike. Trouble free ground starting minimizes the likelihood of aircraft departure delays which engender passenger dissatisfaction and disrupt operations at busy, congested airports. Reliable and successful airstarting of a temporarily disabled engine is obviously desirable, even on multi-engine aircraft, to restore full propulsive power to the aircraft.
Among the factors which can inhibit successful engine starting is circumferential or radial nonuniformity of the fuel-air ratio in the interior of the engine's combustion chamber. For example, the overall ratio of fuel to air in the combustion chamber may be well within the limits for achieving a successful start, but the fuel-air ratio may be excessively lean in the immediate vicinity of the engine ignitors. Nonuniformities in the fuel-air ratio may arise from a number of causes such as uneven distribution of the airstream flowing through the combustion chamber, irregularities in the spray pattern of fuel issuing from the fuel injectors or inadequate atomization of the fuel.
One obvious way to mitigate an inability to start is, of course, to identify and correct the underlying cause of the problem. Unfortunately, the identification of a root cause can require considerable time and effort with no guarantee of success. Even if the cause is accurately determined, it may not be possible to readily implement corrective measures, particularly if those measures include modifications to the fuel injectors, combustion chamber or other internal engine hardware. Such modifications are especially objectionable if the affected hardware is subject to time consuming and costly development and if the hardware must be retrofit into existing engines.
Hardware modifications are also unappealing in view of the common practice among aircraft engine manufacturers of producing multiple, closely related variants or models of engines within a generic engine family. A hardware modification which successfully mitigates an inability to start one engine model within a family may be nonoptimal or completely ineffective for a related model in the same family. Thus, a starting problem which affects multiple models of an engine family may compel the engine manufacturer to develop a number of model specific hardware modifications. The resultant absence of hardware commonality within the family complicates maintenance and repair logistics and is objectionable to the engine's owner.
Another potential solution is to simply replace the exciters, which apply voltage across the ignitors to generate electrical sparks, with exciters of higher capacity so that the ignitor sparks extend further from the ignitors and into a region of the combustion chamber where the fuel-air ratio is more favorable for ignition. Since the exciters are mounted on the exterior of the engine and are readily available in a variety of capacities, this approach may be less objectionable than one which involves modifications or changes to internal engine hardware. However even if the more energetic sparks are capable of reaching a region of adequate fuel-air ratio within the combustion chamber, the expense of higher capacity exciters makes this option unappealing. Higher capacity exciters also add weight and consume additional space--distinct disadvantages in aircraft applications--and may reduce ignitor life.
Another possible strategy is to enrich the fuel-air mixture near one or more of the ignitors during the engine start sequence by distributing a disproportionate share of the engine's fuel to the fuel injectors in the vicinity of the ignitors. Such fuel reapportionment may be effected by changes or adjustments to a fuel metering unit in the engine's fuel system. However, as is the case with other hardware modifications, changes or adjustments to the metering unit introduce undesirable hardware noncommonality into an engine family.
Yet another possible solution is to adjust the behavior of the engine controller, a device which automatically regulates various aspects of engine operation. Gas turbine engine controllers customarily include a starting fuel schedule which governs the quantity of fuel delivered to the combustion chamber during engine starting. Since the starting fuel schedule is typically adjustable, the schedule can be raised to increase the quantity of fuel delivered during an engine start, thereby increasing the likelihood of successful ignition. Such an increase, however, affects the quantity of fuel delivered throughout the entire duration of the start, not just during the ignition phase of the start. Once ignition has occurred, the increased fuel quantity causes the engine to accelerate so rapidly that the aerodynamic stability of the engine compressors is compromised and the engine is unable to complete its acceleration to idle speed.